1. Field of the Invention
This invention relates to a wireless communications and, more particularly, to a band edge power cancellation scheme to reduce interference in a wireless communications system.
2. Description of Related Art
The service area of a wireless communication system is partitioned into connected service domains known as cells, where wireless units communicate via radio links with a base station (BS) serving the cell. The base station is coupled to a land network, for example through a Mobile Switching Center (MSC) which is connected to a plurality of base stations dispersed throughout the service area. In the wireless communications industry, a service provider is often granted two or more non-contiguous or segregated frequency bands to be used for the wireless transmission and reception of RF communications channels. For example, in the United States, a base station for an “A” band provider for cellular communications receives frequency channels within the A (825–835 MHz), A′ (845–846.5 MHz) and A″ (824–825 MHz) bands, and the wireless units receive frequency channels within the A (870–880 MHz), A′ (890–891.5 MHz) and A″ (869–870 MHz) bands. A base station for a B band provider receives frequency channels within the B (835–845 MHz) and B′(846.5–849 MHz) frequency bands, and the wireless units receive frequency channels within the B (880–890 MHz) and B′(891.5–894 MHz) frequency bands. Additionally, a base station for a Personal Communications Systems (PCS) provider may receive frequency channels from wireless units on one or more PCS bands (1850 MHz–1910 MHz), and the wireless units receive frequency channels on one or more PCS bands (1930–1990 MHz).
To improve system performance and increase system capacity, the power levels transmitted by the wireless units and/or the base stations are controlled. Power control is generally done by the receiving unit or station measuring the signal strength from the transmitting station or unit. The receiving unit or station can adjust its transmit power based on the received signal strength, and/or the receiving unit or station can relay power control information to the transmitting unit which adjusts its transmit power level in response to the power control information. The power level transmitted by every wireless unit is typically under the control of the serving base station, and the base station performs power control to reduce the power level that each wireless unit is transmitting while maintaining a good quality reverse link. By decreasing the power level that each wireless unit is transmitting, system-wide interference created by the transmissions of the wireless units is reduced. Such a scenario allows increased capacity for the wireless cellular communications system because as the transmit powers are decreased, the overall signal to interference ratio decreases for all wireless units in the wireless cellular communications system.
Since frequency bands of adjacent cells and/or adjacent wireless communications systems are not under the power control of the same base station, there is a possibility that one or more signals/carriers from another cell that are close to the operating frequency band of a base station might be too strong in power and overload the radio receiver circuitry in the base station. For example, in order to reduce system hardware costs, a service provider would want to use common receivers for the simultaneous reception and processing of signals within the non-contiguous frequency bands. Typically, an automatic gain control (AGC) at the front-end of the receiver is effective in protecting the base station from overload but at the expense of any users at the fringe of the cell.
However, due to the finite roll-off characteristics of filters in the radio receiver, a signal from an adjacent band may come through the radio receiver at a power level strong enough to saturate the wideband analog to digital (A/D) converter. The A/D converter is the most critical component to protect against overload in a modern cellular radio receiver. The A/D converter does not operate in a soft clipping manner as is the case with amplifiers, mixers and other analog semiconductor devices. Once the A/D converter is saturated (i.e., input signal is above the full scale resolution of the A/D), the digital output code cannot go above the maximum binary number limited by the resolution in bits. The sudden change (or sudden stop/clipping) in binary output pattern from the A/D converter, which digitizes and tracks the analog input signal, is called a discontinuity and results in a massive spurious response in the digital domain (when a Fourier transform is taken of the supposed analog input signal with a sudden clipping of the amplitude).
It is necessary to implement some overload protection in order to prevent saturation of the A/D converter.